The present invention relates to electrostatically charged, high-strength, high-modulus, melt-processed microfibers, films having a charged, microfibrillated surface, and methods of making the same. Charged microfibers of the invention can be prepared by imparting fluid energy, typically in the form of high-pressure water jets, to a highly oriented, highly crystalline, melt processed film to liberate microfibers therefrom. Microfibrillated films of the invention find use as tape backings, filters for particulate contaminants, such as face masks and water or air filters, fibrous mats, such as those used for removal of oil from water and those used as wipes, and thermal and acoustical insulation. Microfibers of the invention, when removed from the film matrix may be used in the preparation of woven or nonwoven articles and used as wipes for the removal of debris or dust from a surface.
Polymeric fibers have been known essentially since the beginnings of commercial polymer development. The production of polymer fibers from polymer films is also well known. In particular, the ease with which films produce fibers (i.e., fibrillate) can be correlated to the degree of molecular orientation of the polymer fibrils that make up the film.
Orientation of crystalline polymeric films and fibers has been accomplished in numerous ways, including melt spinning, melt transformation (co)extrusion, solid state coextrusion, gel drawing, solid state rolling, die drawing, solid state drawing, and roll-trusion, among others. Each of these methods has been successful in preparing oriented, high modulus polymer fibers and films. Most solid-state processing methods have been limited to slow production rates, on the order of a few cm/min. Methods involving gel drawing can be fast, but require additional solvent-handling steps. A combination of rolling and drawing solid polymer sheets, particularly polyolefin sheets, has been described in which a polymer billet is deformed biaxially in a two-roll calender then additionally drawn in length (i.e., the machine direction). Methods that relate to other web handling equipment have been used to achieve molecular orientation, including an initial nip or calender step followed by stretching in both the machine direction or transversely to the film length.
Liberating fibers from oriented, high-modulus polymer films, particularly from high molecular weight crystalline films, has been accomplished in numerous ways, including abrasion, mechanical plucking by rapidly-rotating wire wheels, impinging water-jets to shred or slit the film, and application of ultrasonic energy. Water jets have been used extensively to cut films into flat, wide continuous longitudinal fibers for strapping or reinforcing uses. Ultrasonic treatment of oriented polyethylene film in bulk (that is, a roll of film immersed in a fluid, subjected to ultrasonic treatment for a period of hours) has been shown to produce small amounts of microfibrils.
The present invention is directed to novel charged, highly oriented, melt processed polymeric microfibers having an effective average diameter less than 20 microns, generally from 0.01 microns to 10 microns, and substantially rectangular in cross section, having a transverse aspect ratio (width to thickness) of from 1.5:1 to 20:1, and generally about 3:1 to 9:1. Since the microfibers are substantially rectangular, the effective diameter is a measure of the average value of the width and thickness of the microfibers.
The rectangular cross-sectional shape advantageously provides a greater surface area (relative to fibers of the same diameter having round or square cross-section) making the microfibers (and microfibrillated films) especially useful in applications such as filtration and as reinforcing fibers in cast materials. The surface area is generally greater than about 0.25 m2/gram, typically about 0.5 to 30 m2/g. The electrostatic charge imparted to the microfibers (or microfibrillated article) enhances the filtration efficiency. Further, due to their highly oriented morphology, the microfibers of the present invention have very high modulus, for example typically above 109 MPa for polypropylene fibers, making them especially useful as reinforcing fibers in thermoset resin and concrete.
The present invention is further directed toward the preparation of highly-oriented films having a charged microfibrillated surface by the steps of providing a highly oriented, voided, non-conductive semicrystalline polymer film, microfibrillating said voided polymer film by imparting a high pressure fluid jet energy, and thereby imparting a charge to the microfibrillated surface. Surprisingly, it has been found that the microfibrillation of the polymer film imparts an electrostatic charge to the microfibrillated surface without the need for post-microfibrillation processing such as corona discharge treatment. Thus, the present invention provides an efficient process whereby charged microfibers (or microfibrous flakes) may be prepared in one step from highly oriented films by imparting fluid jet pressure to the surface of the film.
The voided film may be a microvoided film, a voided film prepared from an immiscible mixture of a semicrystalline polymer or may be a foam prepared from a high melt strength polymer. As used herein, the term xe2x80x9cfilmxe2x80x9d shall also encompass sheets, including foamed sheets and it may also be understood that other configurations and profiles such as tubes may be provided with a microfibrillated surface with equal facility using the process of this invention.
As used herein, the term xe2x80x9cmicrofibrillated articlexe2x80x9d refers to an articles, such as a film or sheet bearing a microfibrillated surface. The charged, microfibrillated surface may comprise microfibers prepared from uniaxially oriented films, or may comprise a schistose surface of microfibrous flakes, prepared from biaxially oriented films. Optionally the microfibers or microfibrous flakes may be harvested from the microfibrillated surface of the film. For brevity, the term microfiber may be used to refer additionally to the microfibrous flakes.
Advantageously the process of the invention is capable of high rates of production, is suitable as an industrial process and uses readily available polymers. The microfibers and microfibrillated articles of this invention, having extremely small fiber diameter and both high strength and modulus, are useful as tape backings, strapping materials, films with unique optical properties and high surface area, low density reinforcements for thermosets, impact modifiers or crack propagation prevention in matrices such as concrete, and as fibrillar forms (dental floss or nonwovens, for example). When imparted with a charge, by either corona treatment or microfibrillation with high-pressure water jets, the resulting charged microfibers (or microfibrillated articles) are particularly useful in filtration applications, and as wipes for the removal of dusts and other debris from surfaces.